The invention relates to a method and apparatus for measuring height and volume of an object. The method and apparatus of the invention finds application, in particular, to grading objects, such as oysters, by measuring the volume of the object to determine, for example, the volume of oyster meat, by a three-dimensional (3-D) optical volume measurement using multiple laser lines.
Optical sizing systems currently available use a two-dimensional (2-D) image to measure the projected area of an oyster. The actual volume of the object is then estimated based on the 2-D image projection, in a manner known to those skilled in the art.
For example, one conventional method of determining the size of an object, such as an oyster, obtains a 2-D image of the object in order to determine its length and width at various parts of the object. That 2-D image may correspond to a particular amount of light (corresponding to an amount of current) that is picked up by pixels of a charge-coupled device (CCD) camera positioned to receive images from a particular region. Using known characteristics of the color range of an oyster relative to the color of a background surface where the oyster is subjected to light, processing circuitry can readily determine the number of pixels that correspond to the 2-D image of the object.
Once the 2-D image is obtained, the height of the object is obtained by a table lookup procedure. In order to facilitate creating a table for use in a table lookup procedure, image data from many sample objects, such as 500 oysters, is gathered. This data provides 2-D image plots with the corresponding number of pixels illuminated in each respective 2-D image plot. Next, those 500 oyster samples are weighed. With the actual weight data, a relationship between xe2x80x98number of pixels in 2-D image plot of an oysterxe2x80x99 versus xe2x80x98weight of an oysterxe2x80x99 can be obtained, using a linear regression technique, for example. When a 2-D image of a new oyster is scanned, the estimated weight of the oyster can be obtained using the equation relating 2-D image and weight, based on the data obtained from the 500 oysters in the test and setup phase. The lookup table would include a plurality of pairs of data, each of which corresponds to xe2x80x9cweight versus number of pixels in a 2-D imagexe2x80x9d.
Alternatively, the 2-D image plots of the 500 oysters can be obtained using conventional scanning techniques, and the actual volume of each oyster can be determined. One example of determining volume is putting each oyster into a vat of water having a known capacity and measuring the amount of displacement that occurs. Based on this information, a relationship between xe2x80x98number of pixels in 2-D image plot of an oysterxe2x80x99 versus xe2x80x98volume of an oysterxe2x80x99 can be obtained, using a linear regression technique, for example. When a new oyster is scanned to obtain its 2-D image, the estimated volume of the oyster can be obtained by using an equation relating 2-D image and volume, based on the data obtained from the 500 oysters in the test and setup phase. This lookup table would include a plurality of pairs of data, each of which corresponds to xe2x80x9cvolume versus number of pixels in a 2-D imagexe2x80x9d.
For an object such as an oyster, the measurement accuracy of the projected area using the 2-D approach under ideal lighting conditions is approximately 0.75 cm2. This measurement accuracy value is based primarily on camera resolution and field-of-view of the camera. For example, a 256xc3x97256 pixel area-scan covering a 100 cmxc3x97100 cm area will provide 0.69 cm2 area accuracy. The 0.75 cm2 area measurement accuracy contributes to 3.9 cm3 error in volume estimation, which is an error amount determined based on comparing actual volumes with estimated volumes obtained from the 2-D approach. In the case of oysters, this error is almost 20% of the volume of a xe2x80x98selectxe2x80x99 grade oyster. Lighting intensity variations, poor image quality such as glare on the oyster and the background, and background discoloration also contribute significant error to the measurement result.
Moreover, since the height or volume of the oyster is obtained by estimation techniques using similar kinds of objects, there is an inherent measurement error in the process.
As a result, conventional volume measuring approaches for objects such as oysters have significant problems and are not especially accurate.
The invention is directed to an apparatus and a method for accurately estimating a volume of an object, such as an oyster, by using a 3-D optical volume measurement.
The invention may be implemented by a method of measuring a height of a location on an object. The method includes projecting at least one line of radiation from a radiation source onto the object. The method also includes detecting at least one line of radiation reflected from said object. The method further includes comparing the at least one line of reflected radiation with radiation reflected from a reference surface to determine the height of the object at a location corresponding to where the at least one line of radiation impinges on the object.
The invention may also be implemented by a method of measuring a height of a location on an object. The method includes outputting a plurality of laser lines onto a surface area on which the object is currently located, the laser lines forming a plurality of lines on the surface area when no object is located thereon. The method also includes receiving, by a light sensitive device, such as a camera having a plurality of pixel elements that receive light from a plurality of surface locations, respectively, light reflected from the object due to the plurality of laser lines impinging on the object. The method further includes obtaining a binary image of the object based on a light intensity received at each of the plurality of pixels of the light sensitive device, the binary image being used to determine a width and a length of the object at one or more regions of the object. The method still further includes determining a distance that each of the laser lines is displaced relative to a reference location corresponding to a location that said each laser line would be received if no object was currently located in the surface area. The method also includes determining a height of the object based on the displacements previously determined. The method further includes determining the volume of the object based on the determined height, width and length of the object.
The invention may also be implemented by an apparatus for measuring height or volume of an object. The apparatus includes a laser light source that outputs a plurality of laser lines onto a surface area where the object is currently located. The laser lines form a plurality of substantially parallel lines on the surface area when no object is located thereon. The apparatus also includes an image device, such as a camera having a plurality of pixel elements that receive light from a plurality of surface locations. The imaging device receives light reflected from the object as a result of the plurality of laser lines impinging on the object. The apparatus further includes a processing unit which produces a binary image of the object based on a light intensity received at each of the plurality of pixels of the imaging device. The binary image is used to determine width and length of the object. The processing unit determines a distance that each of the laser lines is displaced relative to a reference line. The reference line is formed as a location that corresponds to a location where each laser line would be received if no object was currently located in the surface area. The processing unit determines a height of the object based on the determined distances, and volume of the object based on the determined height, width and length of the object.